1. Field of the Invention
This invention relates to a method for preparing an optical recording medium of phase change type, and more specifically, to a method for initializing such optical recording medium.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical rewritable (or erasable) optical recording medium is of the phase change type wherein a laser beam is directed to the recording layer to change its crystallographic state whereupon a change of reflectivity by the crystallographic change is detected for reproduction of the information. Optical recording media of the phase change type are of great interest since they can be overwritten by modulating the intensity of a single light beam and the optical system of the drive unit used for their operation is simple as compared with magneto-optical recording media.
Most optical recording media of the phase change type used Ge-Te systems which provide a substantial difference in reflectivity between crystalline and amorphous states and have a relatively stable amorphous state. It was also recently proposed to use new compounds known as chalcopyrites.
Chalcopyrite compounds were investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), pp. 228.
Among the chalcopyrite compounds, AgInTe.sub.2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Laid-Open No. (JP-A) 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992.
In addition to these phase change type optical recording media using chalcopyrite compounds, JP-A 267192/1992, 232779/1992, and 166268/1994 disclose phase change type optical recording media wherein an AgSbTe.sub.2 phase forms when a recording layer crystallizes.
In the conventional optical recording medium of phase change type, the recording layer has been formed by such means as vacuum deposition apparatus, and the recording layer immediately after its formation is in non-crystalline state. When the disc having such recording layer is utilized for a rewritable medium, crystallization of the recording layer is generally required and such crystallization is accomplished by a process called initialization.
Various processes have been proposed for the initialization. Typical processes are the process wherein the substrate is heated to crystallization temperature of the recording layer after the deposition of the recording layer (JP-A 3131/1990); the process called "solid phase initialization" wherein the recording layer is crystallized after its deposition by laser beam irradiation (JP-A 366424/1992, 201734/1990, and 76027/1991); a process wherein the substrate having the recording layer deposited thereon is irradiated with a flash light to utilize optical properties of the chalcogen compounds for pseudo-crystallization of the recording layer by photodarkening (JP-A 281219/1992); a process wherein the recording layer is crystallized by means of RF induction heating; a process wherein the substrate is heated simultaneously with the deposition of the recording layer for crystallization (JP-A 98847/1990); a process wherein a dielectric layer is formed as the first layer, and the recording layer is formed on the first layer and heated for crystallization, and the second dielectric layer is formed on the crystallized recording layer (JP-A 5246/1990).
Initialization by laser beam irradiation, however, is a time-consuming process and this process also suffer from insufficient productivity. On the other hand, the process involving the heating of the entire medium prohibited use of inexpensive resin substrates, since the heating during the initialization resulted in deformation of the resin substrate to result in tracking problems. Use of flash light required repeated irradiation to accomplish the crystallization, and productivity was insufficient.
In view of such situation, an apparatus called "bulk eraser" is currently used for the initialization in commercial scale production. A bulk eraser is an apparatus which is capable of irradiating a high power gas laser 6r semiconductor laser beam without tight focusing so that multiple tracks can be crystallized at once. Use of such bulk eraser enables localized heating of the recording layer, and temperature elevation of the substrate is thus avoided to enable the use of a resin substrate of low heat resistance.
In the conventional optical recording medium of phase change type, the medium initialized with a bulk eraser had to be subjected to a step called "priming" before its shipment. The "priming" is a step wherein recording and erasing are repeated for several ten times. The reason for conducting such "priming" is described by referring to FIG. 1B.
FIG. 1B is a schematic view showing crystallinity of the recording layer of phase change type at the stages of immediately after the layer formation, after the initialization, and after the overwriting. In FIG. 1B, crystallinity is represented by the ordinate axis.
The recording layer formed by sputtering is in non-crystalline state. This state is designated non-crystalline state A. When the recording layer of this state is initialized by the conventional method, the recording layer is crystallized. In the conventional initialization, the recording layer is heated for melting, and then allowed to cool slowly. The resulting post-initialization state is designated "crystalline state B". When the thus initialized recording layer is subjected to recording, the recording layer is heated to a temperature near its melting point, and quenched to leave a recording mark in non-crystalline state. Such state is designated "non-crystalline state C". After such recording, the recording layer is heated to a temperature above the crystallization temperature but below the melting temperature for crystallization of the recording mark. This is the erasing upon overwriting. Such post-erasing state is designated "crystalline state D".
In FIG. 1B, the post-initialization crystalline state B and the post-erasing crystalline state D shares the common feature that they are in their crystalline state. The post-initialization crystalline state B and the post-erasing crystalline state D, however, are different in crystallinity, and the crystallinity of the post-initialization crystalline state B is higher than that of the post-erasing crystalline state D. More illustratively, the post-initialization crystalline state B attained by crystallization after melting has a crystal grain size larger than that of the post-erasing crystalline state D attained by crystallization without melting. Such difference in the crystallinity between the region crystallized in the initialization and the region crystallized in the overwriting results in the difference of reflectivity, and it is only after overwriting the entire region of the medium that the reflectivity becomes consistent. In the mark edge recording as used in the overwritable DVD-RAM, such inconsistency in the reflectivity invites misrecognition of the mark edge.
The step of "priming" is conducted to clear such difference in crystallinity between the region crystallized by the initialization and the region crystallized by the erasing.
The "priming", however, is a quite time-consuming process, and productivity is greatly sacrificed by such "priming".
Initialization of the conventional method also suffers from various problems associated with the use of bulk eraser.
A 12 cm optical recording disk usually takes about several ten seconds to several minutes for its initialization with a bulk eraser, and therefore, the process of initialization is the rate-determining step in the production of the optical recording disk.
In the initialization with a bulk eraser, many tracks are initialized at once by using a high power laser beam, and the problems as described below are involved with such bulk initialization.
In such initialization with the bulk eraser, the laser beam is not tightly focused and a beam spot of substantial area is formed. The energy density within the beam spot, however, is not consistent. In addition, in the spiral scanning of the optical recording medium with the beam spot during the initialization, there is some overlap between the annular region scanned by the beam spot in one turn and the annular region scanned in the next turn. Consequently, the reflectivity after the initialization is different from track to track, and the step of "priming" was necessary to clear such difference in the reflectivity, and the inconsistent reflectivity also resulted in the focus servo failure of the driving unit during the "priming". It should also be noted that, in the prepit region of the substrate where unoverwritable pits are preliminarily formed, the reflectivity inconsistency resulting from the initialization can not be cleared by the process of "priming", and the prepit region may have some difficulty in servo and reproduction. In FIG. 1B, the inconsistency in the reflectivity is represented by the hatched region with considerable variation in the degree of crystallinity.
As mentioned above, the beam spot of a bulk eraser is of substantial area, and only limited energy is applied to each track compared to the total beam energy. Typical countermeasures are repeated irradiation of the same track and increase in the laser beam power, which resulted in damages of the recording layer and/or the dielectric layers sandwiching the recording layer, reduced number of overwritable operations, and inconsistent reflectivity. The stress applied to the dielectric layers upon the melting of the recording layer in the course of bulk initialization is larger than the case of single track initialization since multiple tracks, namely, larger area is initialized in the case of the initialization with a bulk eraser. Therefore, the damages of the recording layer and/or the dielectric layers in the case of the initialization with a bulk eraser is serious, and this damage results in the inconsistent reflectivity. In addition, degree of the damage would be different from track to track due to the inconsistent beam intensity within the beam spot and the beam spot overlap as mentioned above. The inconsistency of the reflectivity caused by such damage can not be completely compensated by the process of "priming", and the resulting inconsistency of the reflectivity within the crystallized regions will invite increased jitter and servo failure of the driving unit.